The invention disclosed relates to a satellite telecommunications system featuring multi-beam coverage and dynamically controlled allocation of the satellite transmission capacity.
An important facet of satellite communications is the method of link-up between the satellite and the various earth stations. A person skilled in the art will know that many satellite telecommunications systems developed thus far have been of a multiple access time-division type (TDMA and SS-TDMA) utilizing non-modular switching patterns. In such systems, earth stations can transmit special packages of data, known as bursts, relative to different kinds of media or different services. Only multi-destination bursts exist however, which are of varying length, and generally contain almost the total traffic handled by the emitting station and directed toward a given repeater.
Briefly, with a non-modular switched pattern, each variation in capacity of a given earth station can affect the allocation of bursts emitted from other stations. As there is no beam-pattern password generated on board the satellite in such systems, recourse must be made to complex techniques for its diffusion, as well as to a number of tracking stations needed for continuous monitoring of an onboard clock which governs the beam switching matrix, and to stand-by protocol arrangements that must be implemented in the event of a malfunction occurring at one of the two stations linked.
All communications satellites utilized hither to are therefore "transparent": that is, the signal received is amplified once and has its frequency changed, by a transponder, and is re-transmitted to earth; such satellites cannot therefore be regarded as genuine repeaters located in space.
The system incorporates digital speech interpolation (DSI) which, when transmitting, permits doubling the gain of the transmission capacity. A DSI station transmits a multi-destination burst containing all the traffic between that station and corresponding stations of the same group. When receiving, the DSI station must be able to analyse a given quantity of bursts originating from different sources; the input capacity of a DSI station may be, for instance, 240 voice channels. The concept has since been introduced of allocation on demand within systems operating on DSI, that is, the facility of varying the number of satellite channels allocated to each DSI station.
The introduction of such allocation-on-demand techniques necessarily dictates the use of special protocols, special procedures which must be adopted in allocation of available satellite transmission capacity to the various earth stations. In modern 4-6 GHz and 11-14 GHz systems, the lower frequencies utilized are such as to permit adopting protocols with a lower degree of protection than is necessary at 20-30 GHz, in view of the fact that the attenuating effect of the atmosphere is much reduced. These protocols are different from the procedures used in simple rearrangement of traffic; they can provide frequent re-allocation of transmission capacity to serve the various earth stations, as well as bring about such re-allocation in real time, whereas the latter type are unable to perform either function by reason of their periodic type of operation.
With a non-modular beam-switching pattern it becomes necessary, generally speaking, to make a rearrangement of the entire system for integration even of the smallest variation.
Protocols adopted for the systems mentioned thus far are exclusively of the synchronous type, inasmuch as a variation in the beam-pattern of TDMA systems will affect satellite and earth stations alike; one is therefore faced with limitations, as the system is rendered particularly inflexible. It will be observed moreover, that the method of access generally adopted in modern satellite systems (TDMA) is conditioned by the type of coverage selected. More exactly, one must ensure full interconnection between all antenna beams of a multi-beam coverage system, especially where the number of such beams is particularly great. This requirement has produced the passage from standard TDMA to SS-TDMA: satellite switched time-division, multiple access.
A global coverage system, on the other hand, makes for difficult coordination with other systems and is characterized by low gain of the on-board antenna.
There are also scanning-type systems, but these are extremely complex as a result of high transmission speeds, which call for equally complex apparatus.
It is the object of the invention disclosed herein to eliminate the drawbacks thus outlined which beset systems currently in use, by design and embodiment of a satellite telecommunications system featuring multi-beam coverage and dynamically controlled allocation of the satellite transmission capacity, so as to enable optimum handling of signals transmitted from the various earth stations, wherein regeneration of such signals is brought about on board the satellite and wherein the system utilizes a modular beam switching pattern and asynchronous type protocol procedures.